Field of the Invention
The present invention relates to a measurement apparatus for measuring the shape of an object to be measured, a calculation method, a system, and a method of manufacturing an article.
Description of the Related Art
There is a known measurement apparatus which projects line pattern light such as that of a space coding method onto an object to be measured via a projecting unit such as a projector, and obtains three-dimensional coordinates from the principle of triangulation based on the position of reflected light obtained by an imaging unit. In this measurement apparatus, the measurement result is readily influenced by the material of an object to be measured.
For example, a resin is sometimes handled as an object to be measured in the field of industrial production. When an object to be measured is a resin, light projected onto the object to be measured enters the interior of the object and is scattered in it, that is, so-called internal scattering occurs. If this internal scattering occurs, the reflected light from the object to be measured contains internal scattered light from the interior of the object, in addition to surface scattered light from the surface of the object. Since the internal scattered light contains scattered light at a distance different from that of the surface scattered light, the measurement apparatus calculates a measurement value different from that of the surface position of the object. Therefore, the internal scattered light appears as a systematic error in the measurement apparatus, and decreases the measurement accuracy.
Accordingly, a technique for reducing the influence of internal scattering is proposed in “S. K. Nayer et al. Fast Separation of Direct and Global Components of a Scene Using High Frequency Illumination. SIGGRAPH July, 2006.” (literature 1). In this technique, pattern light including bright and dark portions and having a spatially high frequency is projected onto a resin as an object to be measured, and the intensity distribution in the dark portion containing an internal scattering component is subtracted from the intensity distribution in the bright portion containing a surface scattering component and the internal scattering component. As described above, literature 1 describes that reducing the internal scattering component from the intensity distribution in the bright portion makes it possible to reduce an error (systematic error) by which three-dimensional coordinates obtained by the measurement apparatus systematically shift in the direction of the interior of an object to be measured.
Unfortunately, literature 1 has no practical disclosure concerning the relationship between the spatial frequency of the pattern light and the internal scattering or surface scattering. For example, when the spatial frequency of the pattern light is low, the intensity distribution in the dark portion contains no internal scattering component, so the internal scattering component contained in the intensity distribution in the bright portion cannot properly be removed. On the other hand, when the spatial frequency of the pattern light is high, the bright portion spatially expands when an optical point image intensity distribution containing defocusing is taken into account, so the intensity distribution in the dark portion contains the surface scattering component. If the intensity distribution in the dark portion is subtracted from that in the bright portion, therefore, the surface scattering component is also subtracted from the intensity distribution in the bright portion, so the internal scattering component cannot properly be removed. As described above, literature 1 cannot always optimally reduce the influence of internal scattering in an object to be measured, that is, the systematic error which decreases the measurement accuracy of the measurement apparatus.